An image heating apparatus in which a nip where the recording material is nipped is formed by causing a pressing member to press-contact a heating rotatable member (a fixing roller, a fixing belt, a pressing roller or a pressing belt) to be contacted to the image on the recording material at an outer peripheral surface thereof has been widely used. In a fixing device, which is an example of the image heating apparatus, the recording material on which a toner image is transferred is subjected to heating and pressing in a process in which the recording material is nipped and conveyed in a fixing nip and thus the toner image is melted and squashed to be fixed on the surface of the recording material as the image.
When the fixing speed is intended to be increased in such a fixing device, a lowering in surface temperature of the heating rotatable member by contact with the recording material cannot be remedied only by heating from an inner surface side. Further, it is also required to shorten the time until the surface temperature of the heating rotatable member reaches a necessary temperature after the fixing device is actuated.
For this reason, in Japanese Laid-Open Patent Application (JP-A) 2004-37555, a fixing device has been put into practical use in which a plurality of external heating members which rotate in contact with the outer peripheral surface of the heating rotatable member to directly heat the surface of the heating member are disposed from a recording material contact position along a rotational direction of the heating rotatable member.
Further, at the outer peripheral surface of the heating rotatable member, a parting layer for enhancing a parting property from melted toner is formed and therefore the outer peripheral surface of the heating rotatable member has a high resistance and is liable to be electrically charged by contact with the recording material.
When the fixing speed is intended to be increased in such a fixing device, the contact frequency with the recording material is increased and thus the outer peripheral surface of the heating rotatable member is charged to an excessive potential, so that there is a possibility that an unfixed toner image carried on the recording material is disturbed to lower image quality.
For this reason, JP-A 2002-62752 shows a fixing device constituted so as to discharge (charge-remove) the outer peripheral surface of the heating rotatable member by connecting an external heating member, lowered in resistance value by incorporating an electroconductive substance into a parting layer formed at a surface of the external heating member, to the ground potential.
As shown in JP-A 2002-62752, when the electroconductive substance is incorporated into the parting layer of the external heating member, the parting property of the parting layer is lowered, so that the surface of the external heating member is liable to be contaminated and thus melted toner or the like is liable to be carried on the external heating member surface. As a result, a normal contact state between the external heating member and the heating rotatable member is impaired, so that an original heating performance cannot be achieved.
Particularly, in the case where the plurality of external heating members are provided as disclosed in JP-A 2004-37555, contamination concentrates at an upstream-side external heating member, to which toner deposited on the heating rotatable member in the fixing nip is first contacted, with respect to the rotational direction of the heating rotatable member and therefore the heating performance is impaired in a short period.